Exhaust gas purification system for internal combustion engine

ABSTRACT

There is provided a technology that enables easier adjustment of the temperature of an exhaust gas purification apparatus to a target value by supplying an appropriate quantity of fuel to the exhaust gas purification apparatus even in the case where sub-injection and exhaust addition are performed in an exhaust gas purification system for an internal combustion engine. The system includes a fuel addition valve that adds fuel to the exhaust gas in an exhaust passage upstream of the exhaust gas purification apparatus and unburned fuel discharge unit for discharging gas containing unburned fuel from the combustion chamber of the internal combustion engine to the exhaust passage, wherein when supplying fuel to the exhaust gas purification apparatus to make the temperature of the exhaust gas purification catalyst equal to a target temperature, the quantity of fuel supplied by the fuel addition valve is controlled by a feedback control, and the quantity of fuel supplied by the unburned fuel discharge unit is controlled by an open-loop control.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification system foran internal combustion engine.

BACKGROUND ART

There is a known technology in which a particulate filter (which will besimply referred to as a “filter” hereinafter) is provided in an exhaustpassage of an internal combustion engine so that particulate matter(which will be referred to as “PM” hereinafter) contained in the exhaustgas of the internal combustion engine will not be emitted to theatmosphere.

PM contained in the exhaust gas is once trapped by the filter. As theamount of PM trapped in the filter becomes larger, the pressure of theexhaust gas upstream of the filter will increase, and there is apossibility that a decrease in the output power of the internalcombustion engine and a deterioration in the fuel economy will beinvited thereby. In such cases, the PM deposited on the filter can beremoved by oxidizing it. Such removal of PM trapped on the filter willbe referred to as “regeneration of the filter”.

To perform the regeneration of the filter, a catalyst having anoxidizing ability is provided upstream of the filter, or a catalysthaving an oxidizing ability is arranged to be supported on the filteritself. The regeneration of the filter is performed by supplying fuel tothe catalyst, thereby raising the temperature of the filter.

An NOx storage reduction catalyst (which will be hereinafter referred toas the “NOx catalyst”) stores sulfur components contained in fuel aswell as NOx. The sulfur components thus stored is more difficult to bereleased than NOx and accumulates in the NOx catalyst. This is calledsulfur poisoning. Since the sulfur poisoning causes a decrease in theNOx removal rate of the NOx catalyst, it is necessary to perform asulfur poisoning recovery process for recovery from the sulfur poisoningat an appropriate time. The sulfur poisoning recovery process isperformed by making the temperature of the NOx catalyst high and lettingthe exhaust gas having a theoretical air-fuel ratio or a rich air-fuelratio flow through the NOx catalyst. The temperature of the NOx catalystcan be made high by supplying fuel to the NOx catalyst to raise thetemperature of the NOx catalyst.

Supply of fuel to the catalyst can be achieved by, for example,providing a fuel addition valve in the exhaust passage and adding fuelto the exhaust gas through the fuel addition valve (which will behereinafter referred to as the “exhaust addition” hereinafter). Fuel mayalso be supplied to the catalyst by performing sub-injection(after-injection or post injection) in which fuel is injected againduring the expansion stroke or the exhaust stroke again after performingmain fuel injection in the cylinder.

There is a known technique of controlling the temperature of a catalystand a filter by using both the sub-injection and the exhaust addition(see, for example, patent document 1: Japanese Patent ApplicationLaid-Open No. 2005-83350, patent document 2: Japanese Patent ApplicationLaid-Open No. 2005-83352, and patent document 3: Japanese PatentApplication Laid-Open No. 2005-83351).

In cases where the injection of fuel into the cylinder is performedmultiple times, as is the case with the sub-injection, pulsation of thefuel pressure occurring at the time of the first fuel injection mightsometimes affect the fuel pressure at the time of later fuel injection.For this reason, it is sometimes more difficult to control the fuelquantity precisely in the sub-injection than in the exhaust addition.Furthermore, when the sub-injection is performed, there is a possibilitythat lubricant oil may be diluted due to what is called bore flushing inwhich fuel adheres to the cylinder wall surface to wash away thelubricant oil. Furthermore, if an EGR apparatus is provided, there is apossibility that fuel may enter the EGR apparatus to cause clogging ofan EGR cooler or to make the EGR valve difficult to operate.Furthermore, if the distance from the combustion chamber to the exhaustgas purification apparatus is long, it is difficult to control thesub-injection quantity when regulating the sub-injection quantity inorder to adjust the temperature of the exhaust gas purificationapparatus to a target value, because it takes time for the temperatureof the exhaust gas purification apparatus to change.

However, since fuel supplied by the sub-injection has been vaporized oris in a reactive state, it has an advantage that it readily reacts inthe catalyst.

On the other hand, fuel supplied by the exhaust addition reaches thecatalyst in a liquid state in some cases. Therefore, if only the exhaustaddition is performed, there will sometimes occur HC poisoning in whichHC adheres to the upstream end portion of the catalyst. Furthermore,there is a possibility that fuel that reaches the catalyst in a liquidstate may evaporate in the catalyst to decrease the temperature of theupstream portion of the catalyst. For the above reasons, when theexhaust addition is performed, there is a possibility that clogging inthe upstream end of the catalyst may occur, and/or the overalltemperature of the catalyst may become lower than active temperatures,thereby making regeneration of the filter impossible.

However, the addition of fuel does not causes dilution of lubricant oilunlike with the sub-injection and allows supply of fuel in a wideoperation range irrespective of the operation state of the internalcombustion engine. If the fuel addition valve is provided immediatelyupstream of the exhaust gas purification apparatus, the quantity ofexhaust addition can easily be controlled because the temperature of theexhaust gas purification apparatus changes immediately when the quantityof exhaust addition is regulated in order to adjust the temperature ofthe exhaust purification apparatus to a target value.

As above, the sub-injection and the exhaust addition both haveadvantages and disadvantages. It is difficult to adjust the temperatureof the filter or the NOx catalyst to a target value only by thesub-injection, because it is difficult to control the quantity of fuelprecisely.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above describedproblems, and an object of the present invention is to provide atechnology that enables easier adjustment of the temperature of anexhaust gas purification apparatus to a target value by supplying anappropriate quantity of fuel to the exhaust gas purification apparatuseven in the case where the sub-injection and the exhaust addition areperformed in an exhaust gas purification system for an internalcombustion engine.

To achieve the above object, an exhaust gas purification system for aninternal combustion engine according to the present invention ischaracterized by comprising:

an exhaust gas purification apparatus that is provided in an exhaustpassage of the internal combustion engine and of which a purificationcapability is recovered by supply of fuel thereto;

a fuel addition valve that adds fuel to exhaust gas in the exhaustpassage upstream of said exhaust gas purification apparatus; and

unburned fuel discharge means for discharging gas containing unburnedfuel from a combustion chamber of said internal combustion engine to theexhaust passage,

wherein when supplying fuel to said exhaust gas purification apparatusto make the temperature of the exhaust gas purification catalyst equalto a target temperature, the quantity of fuel supplied by said fueladdition valve is controlled by a feedback control, and the quantity offuel supplied by said unburned fuel discharge means is controlled by anopen-loop control.

The temperature of the exhaust gas purification apparatus is raised bysupply of fuel at the time when its purification capability isrecovered. The time when the purification capability is recovered mayincludes, for example, the time when NOx stored in an NOx catalyst isreduced, the time when recovery from sulfur poisoning of the NOxcatalyst is performed, and the time when particulate matter deposited ona particulate filter is oxidized (namely the time when regeneration ofthe filter is performed). When the purification capability of theexhaust gas purification apparatus is recovered, fuel is supplied to theexhaust gas purification apparatus through the fuel addition valveand/or fuel is supplied to the exhaust gas purification apparatusthrough the unburned fuel discharge means. The supply of fuel throughthe fuel addition valve and the supply of fuel through the unburned fueldischarge means may be performed at the same time, or only either one ofthem may be performed.

It is difficult to adjust the quantity of fuel supplied through theunburned fuel discharge means to a target value, because it isinfluenced by pulsation of the fuel pressure. In other words, even if afeedback control of the fuel supply quantity is performed by sensing thetemperature of the exhaust gas purification apparatus so as to make thetemperature close to a target temperature, it is difficult to adjust thequantity of fuel supplied by the unburned fuel discharge meansprecisely. Therefore it is difficult to adjust the temperature to thetarget value by a feedback control.

In contrast to this, according to the present invention, the quantity offuel supplied by the unburned fuel discharge means is controlled by anopen loop control. Namely, a predetermined quantity of fuel is supplied.This predetermined quantity may be changed in accordance with theoperation state of the internal combustion engine (e.g. the engine speedor the engine load).

On the other hand, the quantity of fuel supplied through the fueladdition valve can be adjusted more precisely as compared to thequantity of fuel supplied through the unburned fuel discharge means.Therefore, it is possible to correct the fuel supply quantity byfeedback-controlling the quantity of fuel supplied through the fueladdition valve. Consequently, the temperature of the exhaust gaspurification apparatus can be adjusted to the target value quickly.

According to the present invention, said exhaust gas purificationapparatus is provided with a catalyst, and the supply of fuel by saidunburned fuel discharge means may be stopped when the temperature of thecatalyst is not lower than an active temperature.

Since the fuel supplied into the combustion chamber is exposed to hightemperature combustion gas, most part of the fuel is in a vaporizedstate or reformed state as it is discharged from the internal combustionengine. In consequence, the fuel discharge from the internal combustionengine is apt to react in the exhaust gas purification apparatus.

On the other hand, the fuel added through the fuel addition valve isadded to the exhaust gas having a temperature lower than the temperaturein the interior of the combustion chamber, and therefore it is harder tobe vaporized as compared to the fuel supplied into the cylinder.Therefore, it sometimes reaches the exhaust purification apparatus in aliquid state. Even if fuel is vaporized, the exhaust gas in which theair-fuel ratio is locally low due to insufficient diffusion of fuelreaches the exhaust gas purification apparatus, in some cases. In suchcases, it is difficult to raise the temperature of the exhaust gaspurification apparatus by adding fuel through the fuel addition valve.

In the meantime, the supply of fuel through the unburned fuel dischargemeans may cause bore flushing and/or invite deterioration in the fueleconomy. On the other hand, if the temperature of the catalyst is notlower than the active temperature, even the fuel supplied through thefuel addition valve can react in the catalyst. Therefore, when thetemperature of the catalyst is not lower than the active temperature,the temperature of the exhaust gas purification apparatus can be raisedwhile preventing bore flushing from occurring, by stopping the supply offuel through the unburned fuel discharge means and performing only thesupply of fuel through the fuel addition valve. When the temperature ofthe catalyst is lower than the active temperature, the supply of fuelthrough the fuel addition valve and the supply of fuel through theunburned fuel discharge means may be performed at the same time, or onlythe supply of fuel through the unburned fuel discharge means may beperformed.

According to the present invention, fuel may be supplied through saidunburned fuel discharge means at predetermined intervals even while fuelis supplied through said fuel addition valve.

If fuel is supplied only through the fuel addition valve, fuel mayadhere to the upstream end portion of the exhaust gas purificationapparatus, or the temperature of the upstream end portion may decrease,in some cases. This leads to the occurrence of clogging in the upstreamend portion and/or a decrease in the overall temperature of the exhaustgas purification apparatus, in some cases. In contrast, if the supply offuel by the unburned fuel discharge means is performed periodically,reactive fuel can be supplied to the exhaust gas purification apparatus,and therefore temperature of the upstream end portion can be raised.This can prevent clogging in the upstream end portion of the exhaust gaspurification apparatus and a decrease in the overall temperature of theexhaust gas purification apparatus from occurring.

Said predetermined interval may be equal to, for example, time taken forthe amount of fuel added through the fuel addition valve and adhering tothe exhaust gas purification apparatus to exceed a threshold, or apredetermined specific time period. The length of the predeterminedinterval may be changed in accordance with the operation state of theinternal combustion engine.

As described in the foregoing, according to the present invention, in anexhaust gas purification system for an internal combustion engine,easier adjustment of the temperature of an exhaust gas purificationapparatus to a target value can be achieved by supplying an appropriatequantity of fuel to the exhaust gas purification apparatus even in thecase where the sub-injection and the exhaust addition are performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the general configuration of an internalcombustion engine to which an exhaust gas purification system for aninternal combustion engine according to an embodiment is applied and itsair-intake system and exhaust system.

FIG. 2 is a flow chart showing a flow of a temperature raising control,during regeneration of a filter in the embodiment.

THE BEST MODE FOR CARRYING OUT THE INVENTION

In the following, a specific embodiment of the exhaust gas purificationsystem for an internal combustion engine according to the presentinvention will be described with reference to the drawings.

Embodiment 1

FIG. 1 is a diagram showing the general configuration of an internalcombustion engine 1 to which an exhaust gas purification system for aninternal combustion engine according to this embodiment is applied andits air-intake system and exhaust system. The internal combustion engine1 shown in FIG. 1 is a water-cooled four-stroke-cycle diesel engine.

The internal combustion engine 1 is connected with an intake passage 2and an exhaust passage 3. A throttle 4 is provided in the middle of theintake passage 2. The throttle 4 is opened/closed by an electricactuator. An air flow meter 5 that outputs a signal indicative of theflow rate of the intake air flowing in the intake passage 2 is providedin the intake passage 2 upstream of the throttle 4. The quantity offresh intake air of the internal combustion engine 1 is measured by thisair flow meter 5.

On the other hand, in the middle of the exhaust passage 3, there areprovided, in order from the internal combustion engine 1 side, a firstoxidation catalyst 15, a second oxidation catalyst 16, and a particulatefilter 6 (which will be hereinafter referred to as the “filter 6”). Thefilter 6 may support an oxidation catalyst, a three-way catalyst, or anNOx storage reduction catalyst. In this embodiment, the second oxidationcatalyst 16 and the filter 6 correspond to the exhaust gas purificationapparatus according to the present invention.

Furthermore, in this embodiment, a fuel addition valve 7 that adds fuel(light oil) serving as a reducing agent to the exhaust gas flowing inthe exhaust passage 3 downstream of the first oxidation catalyst 15 andupstream of the second oxidation catalyst 16 is provided. The fueladdition valve 7 is opened by a signal from an ECU 20, which will bedescribed later, to inject fuel. The fuel injected through the fueladdition valve 7 into the exhaust passage 3 flows into the secondoxidation catalyst 16 together with the exhaust gas flowing fromupstream in the exhaust passage 3 and is oxidized in the secondoxidation catalyst 16. The temperature of the filter 6 is raised by theheat of reaction generated thereby.

The internal combustion engine 1 is provided with an EGR apparatus 8that recirculates a portion of the exhaust gas flowing in the exhaustpassage 3 to the intake passage 2. The EGR apparatus 8 includes an EGRpassage 81 and an EGR valve 82.

The EGR passage 81 connects the exhaust passage 3 upstream of the firstoxidation catalyst 15 and the intake passage 2 downstream of thethrottle 4. The recirculation of exhaust gas is achieved by the exhaustgas flow through the EGR passage 81. The EGR valve 82 adjusts the crosssectional area of the channel of the EGR passage 81, thereby regulatingthe quantity of EGR gas flowing in the EGR passage 81.

A downstream temperature sensor 9 that outputs a signal indicative ofthe temperature of the exhaust gas flowing in the exhaust passage 3 andan air fuel ratio sensor 10 that outputs a signal indicative of theair-fuel ratio of the exhaust gas flowing in the exhaust passage 3 areattached to the exhaust passage 3 downstream of the filter 6. Thetemperature of the filter 6 is measured based on the output signal ofthe downstream temperature sensor 9. Furthermore, an upstreamtemperature sensor 17 that outputs a signal indicative of thetemperature of the exhaust gas flowing in the exhaust passage 3 isattached to the exhaust passage 3 between the first oxidation catalyst15 and the second oxidation catalyst 16.

Furthermore, the internal combustion engine 1 is provided with a fuelinjection valve 11 that supplies fuel into a cylinder of the internalcombustion engine 1.

An ECU 20 or an electronic control unit for controlling the internalcombustion engine 1 is annexed to the internal combustion engine 1having the above-described configuration. The ECU 20 is a unit thatcontrols the operation state of the internal combustion engine 1 inaccordance with operation conditions of the internal combustion engine 1and requests of the driver.

The ECU 20 is connected, through electrical wiring, with an acceleratoropening degree sensor 13 that outputs an electrical signal indicative ofthe amount of operation of the accelerator pedal 12 depressed by thedriver to enable detection of the engine load and a crank positionsensor 14 that senses the engine speed, in addition to the abovedescribed sensors. Output signals of these various sensors are input tothe ECU 20. The ECU 20 is also connected with the fuel injection valve11 and the fuel addition valve 7 through electrical wiring. Theopening/closing times of the fuel injection valve 11 and the fueladdition valve 7 are controlled by the ECU 20.

In this embodiment, when regeneration of the filter is performed, thetemperature of the exhaust gas is raised by supplying fuel to the secondoxidation catalyst 16, and the exhaust gas whose temperature has beenthus raised is caused to flow through the filter 6, thereby raising thetemperature of the filter 6. The supply of fuel is stopped at the timewhen the temperature of the filter 6 reaches a temperature that isneeded for oxidation of PM, thereby maintaining the temperature of thefilter 6. Since the exhaust gas contains oxygen even during the time inwhich the temperature of the filter 6 is raised, the PM trapped in thefilter 6 is oxidized.

When the sulfur poisoning recovery process for an NOx catalyst isperformed in the case where the NOx storage reduction catalyst issupported on the filter 6 also, the temperature of the filter 6 israised to a temperature needed for recovery from sulfur poisoning bysupplying fuel to the second oxidation catalyst 16 in a similar manner.Thereafter, the sulfur poisoning recovery control is performed bysupplying fuel intermittently.

Such supply of fuel to the second oxidation catalyst 16 can be achievedby injecting fuel through the fuel addition valve 7 and/or by decreasingthe air-fuel ratio of the exhaust gas discharged from the internalcombustion engine 1. In other words, regeneration or recovery fromsulfur poisoning of the filter 6 can be performed by supplying fuel tothe second oxidation catalyst 16 by these means.

Gas containing fuel can be discharged from the internal combustionengine 1 by performing sub-injection (after-injection or post injection)in which fuel is injected through the fuel injection valve 11 during theexpansion stroke or the exhaust stroke again after performing main fuelinjection. At the same time, the intake air quantity may be decreased,and/or the EGR gas quantity may be increased. In this embodiment, theECU 20 that performs the sub-injection corresponds to the unburned fueldischarge means according to the present invention.

The intake air quantity can be decreased by operating the throttle 4 inthe closing direction. The EGR gas quantity can be increased byoperating the EGR valve 82 in the opening direction.

Hereinafter, the injection of fuel through the fuel addition valve 7will be referred to as the “exhaust addition”.

When the main fuel injection for generating torque is performed throughthe fuel injection valve 11, pulsation of the fuel pressure can occurdue to a sudden decrease in the pressure of fuel supplied to the fuelinjection valve 11. If the sub-injection is performed immediately afterthe main injection, the injection quantity will sometimes becomeunstable because the fuel injected by the sub-injection is affected bypulsation of the fuel pressure caused by the main injection, in somecases. In such cases, even if a feedback control that adjusts thesub-injection quantity so as to make the temperature measured by thedownstream temperature sensor 9 close to a target temperature isperformed, it might be difficult to make the measured temperature closeto the target temperature.

On the other hand, the exhaust addition is performed at intervals longerthan the intervals of the main injection and the sub-injection by thefuel injection valve 11 at a low fuel pressure. Therefore, the exhaustaddition is hard to be affected by pulsation of the fuel pressure.Therefore, when a feedback control that adjusts the quantity of exhaustaddition so as to make the temperature measured by the downstreamtemperature sensor 9 close to a target temperature is performed, themeasured temperature can easily be made close to the target temperature.

In view of the above, in this embodiment, the sub-injection quantity isadjusted by an open loop control, and the exhaust addition quantity isadjusted by a feedback control.

Since the fuel supplied by the sub-injection is vaporized by the heat ofthe gas burned in the combustion chamber, it easily reacts in the secondoxidation catalyst 16. On the other hand, when the sub-injection isperformed, fuel will adhere to the cylinder wall surface, therebydiluting lubricant oil due to mixing of the lubricant oil and fuel.Furthermore, if the fuel ejected to the exhaust passage 3 by thesub-injection flows into the EGR passage 81 and adheres to the EGR valve82, there is a possibility that the EGR valve 82 may become difficult tooperate.

On the other hand, since the exhaust addition can be performed withlittle influence of the load of the internal combustion engine 1, it canbe performed in a wider operation range than the sub-injection. However,there is an operation range in which the NOx reduction efficiency isdeteriorated due to difficulty in diffusion of fuel and/or adhesion offuel to the wall surface of the exhaust passage 3.

In view of this, in this embodiment, when the temperature of thecatalyst to which the fuel supplied by the exhaust addition reachesfirst (that is, in this embodiment, the second oxidation catalyst 16)reaches an active temperature, namely a temperature at which thereaction of fuel can be achieved satisfactorily (e.g. 25° C. to 300°C.), the sub-injection is not performed, but only the exhaust additionis performed. Specifically, the exhaust addition is performed if fuelcan react in the second oxidation catalyst 16. Thus, the fuel reacts inthe second oxidation catalyst 16, and the temperature of the filter 6can be raised. Not performing the sub-injection can prevent the boreflushing.

The sub-injection may be performed at regular intervals even while thetemperature of the filter 6 is being raised or maintained by the exhaustaddition as described above. Even after the temperature of the secondcatalyst 16 has reached the active temperature, there is a possibilitythat HC poisoning may occur while the exhaust addition is performed dueto adhesion of liquid fuel to the upstream end portion of the secondoxidation catalyst 16. In addition, there is a possibility that thetemperature of the upstream end portion of the second catalyst 16 maybecome lower than the active temperature with the vaporization of thefuel adhering to the upstream end portion of the second catalyst 16. Ifthis occurs, there is a possibility that clogging may occur in theupstream end portion of the second catalyst 16, and/or the overalltemperature of the second oxidation catalyst 16 may become lower thanthe active temperature.

In contrast, if the sub-injection is performed periodically, fuel havinga high reactivity can be supplied to the second oxidation catalyst 16periodically, and therefore a decrease in the temperature of the secondoxidation catalyst 16 can be prevented. Furthermore, by the exhaustaddition, the fuel adhering to the upstream end portion of the secondoxidation catalyst 16 can be removed by reaction. The time at which thesub-injection is performed may be the time at which clogging of thesecond catalyst 16 can occur, which may be determined in advance by, forexample, an experiment. Alternatively, the amount of HC poisoning of thesecond catalyst 16 may be estimated based on the exhaust additionquantity, the flow rate of the exhaust gas, and the temperature of theexhaust gas etc, and the sub-injection may be performed at the amount ofHC poisoning reaches a threshold. Still alternatively, the sub-injectionmay be performed at predetermined time intervals.

Next, a temperature raising control during the regeneration of thefilter 6 in this embodiment will be described. FIG. 2 is a flow chartshowing a flow of the temperature raising control during theregeneration of the filter 6 in this embodiment. This routine isexecuted repeatedly at predetermined time intervals during theregeneration of the filter 6. A similar temperature raising control maybe performed also during the sulfur poisoning recovery for the NOxcatalyst.

In step S101, the temperature of the exhaust gas is raised. In thisstep, the temperature of the first oxidation catalyst 15 is raised. Ifthe temperature of the first oxidation catalyst 15 is raised to anactive temperature, the fuel supplied by the sub-injection can react inthe first oxidation catalyst 15. Since the volume of the first oxidationcatalyst 15 is small as compared to the second oxidation catalyst 16, aportion of the fuel supplied by the sub-injection flows downstreamwithout reacting, even if the oxidation catalyst 15 is at an activetemperature.

In this step, the delayed injection in which the timing of the maininjection by the fuel injection valve 11 is delayed or theafter-injection is performed, thereby burning fuel in the combustionchamber to raise the temperature of the exhaust gas. The temperature ofthe exhaust gas can be further raised by closing the EGR valve 82 and/orclosing the throttle 4, at the same time. In cases where the system isequipped with an exhaust throttle valve, the opening degree of theexhaust throttle valve may be adjusted. These components may becontrolled based on a map related to the engine speed and the engineload.

In step S102, a determination is made as to whether or not thetemperature of the exhaust gas flowing into the second oxidationcatalyst 16 is equal to or higher than e.g. 250° C. This temperature isa temperature at which the fuel supplied by the exhaust addition canreact in the second oxidation catalyst 16, which is, for example, anactive temperature of the second oxidation catalyst 16. The temperatureof the exhaust gas can be measured by the upstream temperature sensor17.

If the determination in step S102 is affirmative, the process proceedsto step S104. On the other hand, if the determination in step S102 isnegative, the process proceeds to step S103.

In step S103, the post injection is performed. The quantity of fuelinjected by the post injection is controlled by an open loop control. Ifthe first oxidation catalyst 15 is at an active temperature, a portionof the fuel discharged by the post injection reacts in the firstcatalyst 15, and therefore the temperature of the exhaust gas is raised.The ECU 20 determines the open timing and the open time of the fuelinjection valve 11 in such a way that a fuel injection quantity that hasbeen determined in relation to, for example, the engine speed and theengine load in advance by an experiment is achieved. Then, the processreturns to step S102.

In step S104, the exhaust addition is performed. In this step, since thetemperature of the second oxidation catalyst 16 has been raised enough,the fuel supplied by the exhaust addition reacts in the second oxidationcatalyst 16. The exhaust addition quantity at this time is determinedbased on the required heat value. The required heat value is determinedbased on how much heat is to be generated by the exhaust addition toachieve the target temperature. This is determined based on thetemperature of the exhaust gas measured by the upstream temperaturesensor 17, the heat capacities of the second oxidation catalyst 16 andthe filter 6, and the flow rate of the exhaust gas.

A feedback control for adjusting the exhaust addition quantity isperformed in such a way that the temperature of the filter 6 measured bythe downstream temperature sensor 9 becomes equal to a targettemperature. The target temperature is a temperature needed for theregeneration of the filter 6. At this time, the exhaust additionquantity is corrected by learning, and in the subsequent occasions ofthe exhaust addition, the exhaust addition quantity corrected bylearning is used. If the sub-injection controlled by an open loopcontrol and the exhaust addition controlled by a feedback control areperformed at the same time, the exhaust addition quantity is influencedby changes in the sub-injection quantity. However, since the responsetime of the exhaust addition from the addition of fuel to the rise ofthe temperature of the filter 6 is short, the temperature of the filter6 can be controlled satisfactorily by the exhaust addition. Although thelearning control of the exhaust addition quantity is also influenced bychanges in the sub-injection quantity, the influence can be decreased byperforming the learning over a long period of time or multiple times.

As described in the foregoing, according to the embodiment, since afeedback control is used when performing the exhaust addition while notused when performing the sub-injection, the temperature of the filter 6can easily be adjusted to a target temperature. Furthermore, since thesub-injection is stopped after the temperature of the second oxidationcatalyst 16 has risen to, for example, an active temperature, dilutionof lubricant oil by fuel can be prevented. Still further, by performingthe sub-injection periodically even after the temperature of the secondoxidation catalyst 16 has become equal to or higher than, for example,the active temperature, a decrease in the temperature of the secondoxidation catalyst 16 and clogging of the second oxidation catalyst 16can be prevented from occurring.

1. An exhaust gas purification system for an internal combustion engineby comprising: an exhaust gas purification apparatus that is provided inan exhaust passage of the internal combustion engine and includes anoxidation catalyst and of which a purification capability is recoveredby supply of fuel thereto; a fuel addition valve that adds fuel toexhaust gas in the exhaust passage upstream of said exhaust gaspurification apparatus; and unburned fuel discharge unit for discharginggas containing unburned fuel from a combustion chamber of said internalcombustion engine to the exhaust passage, wherein when supplying fuel tosaid oxidation catalyst by at least one of said fuel addition valve andsaid unburned fuel discharge unit to make the temperature of the exhaustgas purification apparatus equal to a target temperature upon recoveringthe purification capability of the exhaust gas purification apparatus,the quantity of fuel supplied by said fuel addition valve is controlledby a feedback control, and the quantity of fuel supplied by saidunburned fuel discharge unit is controlled by an open-loop control. 2.An exhaust gas purification apparatus for an internal combustion engineaccording to claim 1, wherein when the temperature of the oxidationcatalyst is not lower than an active temperature, supply of fuel by saidunburned fuel discharge unit is stopped.
 3. An exhaust gas purificationsystem for an internal combustion engine according to claim 1, whereinfuel is supplied at predetermined intervals by said unburned fueldischarge unit even while fuel is supplied by said fuel addition valve.4. An exhaust gas purification system for an internal combustion engineaccording to claim 2, wherein fuel is supplied at predeterminedintervals by said unburned fuel discharge unit even while fuel issupplied by said fuel addition valve.